The present invention relates to a method and an apparatus for manufacturing batteries, and particularly to a method and an apparatus for welding current collector plates to electrode plate groups.
For the structure of rechargeable batteries such as nickel metal hydride rechargeable batteries, a prismatic structure has been proposed, wherein an electrode plate group 21 as shown in FIG. 3A is accommodated together with a liquid electrolyte within a prismatic case (not shown) having a rectangular cross-section and an open top end which is closed with a lid member (not shown). Such structure serves to increase the electrode surface area as much as possible within a restricted space, and in addition to raising the reactive efficiency of the electrode plates, thereby enabling a large current to be taken. It therefore significantly affects battery characteristics.
More specifically, as shown in FIG. 3B in detail, the electrode plate group 21 is constituted by alternately superimposing a plurality of positive electrode plates 22 made of foamed nickel and a plurality of negative electrode plates 23 formed of punched metal with an active material of hydrogen-absorption alloy powder in paste form, each of the positive electrode plates 22 being respectively covered with bag-shaped separators 24 made of polypropylene non-woven cloth having openings on one side, so that the positive and negative electrode plates 22, 23 are layered upon one another with intervening separators 24 therebetween.
The plurality of positive electrode plates 22 and the plurality of negative electrode plates 23 respectively have their lead portions 22a, 23a on one side, that are protruded outwards on the opposite sides. The positive electrode lead portions 22a are formed by compressing the foamed nickel and seam-welding lead plates on one side thereof by ultrasonic welding, while the negative electrode lead portions 23a are constructed of part of the electrode plates which is left uncoated with the active material. A positive electrode current collector 25 and a negative electrode current collector 26, both made of either a nickel sheet or a nickel-plated steel sheet, are abutted perpendicularly to the side edges of the lead portions 22a, 23a on the opposite sides of the electrode plate group 21, and joined thereto by welding.
Welding methods using electronic beams have been proposed for the joining of the collector plates 25, 26 to the lead portions 22a, 23a. One such example is illustrated in FIG. 4 and FIGS. 5A and 5B. As shown, a plurality of corrugations 27 (seven in the illustrated example) are formed in the collector plates 25, 26 preliminarily at certain spaces in the longitudinal direction, and solder material 28 such as nickel solder is applied within the elongated indentations of the corrugations 27 on the side contacting the lead portions 22a, 23a. Under a state wherein these collector plates 25, 26 are tightly pressed against the lead portions 22a, 23a, electronic beams 30 are irradiated on the backside of the collector plates 25, 26 opposite from the side contacting the lead portions 22a, 23a in a vacuum atmosphere. The electronic beams 30 are scanned in the direction in which the electrode plates are layered as indicated by the arrow, whereupon the collector plates 25, 26 are heated and the solder material 28 melts. The collector plates 25, 26 are thus welded to the side edges of the lead portions 22a, 23a. This welding action is performed to the plurality of locations in the lengthwise direction of the collector plates simultaneously or successively. The lead portions 22a, 23a are formed with a pair of position locating holes 29a through which corresponding position locating pins 29 are passed whereby the side edges of the lead portions 22a, 23a are aligned to form flat planes, so that the welding of the collector plates 25, 26 can be favorably performed.
The production equipment for manufacturing such electrode plate groups includes a processing chamber capable of vacuum exhaustion, in which electronic beam irradiating means are arranged. Electrode plate groups 21, to which collector plates 25, 26 have been assembled, are carried into this processing chamber, and when a vacuum is drawn to a desired degree, the welding operation is performed. Thereafter, the pressure within the processing chamber is returned to an atmospheric level, whereupon the processed electrode plate group 21 is removed, and next electrode plate group is carried in.
There can also be an arrangement, wherein a preliminary chamber and a post-processing chamber are respectively arranged adjacent the processing chamber, the former being evacuated to the vacuum level of the processing chamber during the welding of the preceding electrode plate group, and the latter being arranged to return the pressure therein from the vacuum to the atmospheric level.
Japanese Laid-open Patent Application No. 53-114748 discloses an electronic beam welding apparatus applicable to mass-production of one type of works, wherein a plurality of chambers having an identical constitution are arranged in line for cyclic processes. An automatic attachment/removal mechanism is provided for successively connecting a single electronic beam irradiating means to each of the chambers corresponding to the welding process in each chamber.
The problem faced by the method wherein electrode plate groups are carried into a single processing chamber and the processing chamber is then vacuumed is that a considerably long time is required until a vacuum of desired level is drawn. Even if the vacuuming speed is increased, because of the water which is much contained within the positive electrode plates 22 and which is released gradually as time passes, the evacuation needs to be carried on for a long time until a vacuum of more than a predetermined level is created. Thus the production efficiency is extremely poor, since it took time until the welding can actually be started after the electrode plate group has been introduced into the processing chamber. Also, the cost is considerably high, and so this method was hardly applicable to mass-production.
If a multiplicity of processing chambers were provided and welding performed at the same time in these chambers, mass-production could be possible. However, such equipment with a large number of processing chambers would be horrendously expensive, hence impracticable.
In terms of time consumed for creating a high vacuum, it is also the case with a system wherein a preliminary chamber and a post-processing chamber are arranged adjacent the processing chamber, since the production tact time in such system is known to be as long as about 300 sec. In order to realize mass-production, the tact time must be reduced to about 50 sec.
The apparatus disclosed in the above-mentioned Japanese Laid-open Patent Application No. 53-114748 provides no solution to the problem that it takes time for drawing a vacuum when handling a work which contains much water such as the electrode plate group. The system requires a plurality of expensive chambers capable of drawing a high vacuum, and a complex structure for switchably connecting the electronic beam irradiating means to each of the chamber, hence the equipment cost is extremely high.
Next, another problem encountered in prior arts is described referring to FIG. 10A, which is a schematic illustration of a prior art welding method. Normally, when welding the collector plates and the electrode plate groups together by scanning an electronic beam in a direction in which the electrode plates are layered, the signal waveform for the scanning of the electronic beam is triangular, in order to make the scanning speed constant so as to apply heat uniformly along the scanning direction.
However, when applying heat to a collector plate 122 by scanning the electronic beam 123 along the widthwise direction of the collector plate, both side edges of the irradiated portion 124 receives less heat than a middle part because there is no heat applied on their outer sides and heat is more readily dissipated. As a result, there occurred a problem of incomplete welding because part of the collector plate 122 was left unmelted. If the output or time for irradiation of electronic beam was increased to augment the entire amount of heat, the heat applied to the middle part of the collector plate 122 would become excessive and it would only lead to another problem that separators interposed between electrode plates are damaged, which causes short-circuiting between the electrode plates.
In view of these problems, additional scannings 126, 127 are usually performed once or a plurality of times in spots or over a very short distance at both side edges of the collector plate 122 in addition to the overall scanning 125 over the entire width of the collector plate 122, as shown in FIGS. 10A and 10B. According to such method, however, since the additional scannings 126, 127 are required at both side edges, which take 35 ms and 25 ms respectively, in addition to the scanning 125 of the entire width of the collector plate 122 which takes 90 ms, as illustrated in FIG. 10B, the scanning time necessary for welding one line sums up to 210 ms. If a collector plate 122 is to be joined to an electrode plate group 121 at 5 locations, it takes 1050 ms. Thus the conventional welding process is very time-consuming, which is partly the cause of low productivity in the manufacture of electrode plate groups for batteries.